Method for increasing the output of an electron accelerator

ABSTRACT

A method of increasing the energy output of an electron accelerator by scanning the electron beam produced in the accelerator in a manner so that excessive heat build-ups do not occur at the electron permeable window structure through which the beam passes to exit the accelerator. This method is used in conjunction with an accelerator having at least two elongate, electron permeable windows arranged in a side-by-side orientation and scanning coils allowing the beam to be scanned parallel to at least two, perpendicular axes. The method provides that the beam is scanned along the length of a first of the windows in a first direction and then instantaneously is scanned laterally to the second of the windows. The beam then is scanned along the length of the second of the windows in a second direction 180* from the first direction. A return of the beam to its original position is accomplished as the beam then is scanned laterally and instantaneously a second time such that it again is directed at the first of the windows. Unlimited repetition of these scanning steps is possible.

United States Patent [151 3,679,930 Colvin et al. 51 July 25, 1972 [54]METHOD FOR INCREASING THE Primary Examiner-Rodney D. l3ennett, Jr.

OUTPUT OF AN ELECTR N Assistant Examiner-Brian L. Ribando ACCELERATOR OAtt0rneyJohn R. Faulkner and E. Denms OConnor [57] ABSTRACT [72]Inventors: Alex D. Colvin, Livonia; David H. McNltt,

Royal Oak; Allen Turner Ann Arbor A method of increasing the energyoutput of an electron acan of Mich. celerator by scanning the electronbeam produced in the accelerator in a manner so that excessive heatbuild-ups do not occur at the electron permeable window structurethrough [73] Asslgnee' Ford Motor Company Dearbom Mlch' which the beampasses to exit the accelerator. This method is [22] Filed; Aug 3 19 9used in conjunction with an accelerator having at least two elongate,electron permeable windows arranged in a side-by- PP 849,705 sideorientation and scanning coils allowing the beam to be scanned parallelto at least two, perpendicular axes. The method provides that the beamis scanned along the length of [52] US. Cl. ..315/21 R, 315/24 3 firstof the windows in a first direction and then instantane- [51] Int. Cl...H01 j 29/76 ously is scanned laterally to the second of the windows.The [58] Field of Search ..3l5/l8, 24, 26, 31, 21 beam then is scannedalong the length of the second of the windows in a second direction 180from the first direction. A return of the beam to its original positionis accomplished as [56] Refe'emes cued the beam then is scannedlaterally and instantaneously a second time such that it again isdirected at the first of the win- UNITED STATES PATENTS dows. Unlimitedrepetition of these scanning steps is possible. 3,066,238 11/1962 Arndt..3l5/22 6 Claims, 4 Drawing Figures METHOD FOR INCREASING THE OUTPUT OFAN ELECTRON ACCELERATOR BACKGROUND OF THE INVENTION Electron beamgenerators or electron accelerators having accelerating voltages in theorder of several million volts conventionally include a long insulatingcontainer. This container defines an evacuated chamber through whichelectrons are accelerated to form a beam by means of a large potentialdifference existing between an electron gun, including a hot cathodeemitter at one end of the chamber, and an anode at the other end of thechamber. The anode includes an electron permeable window through whichthe beam passes from the chamber onto a substance being irradiated.

Conventional materials for the formation of electron permeable windowsare thin metal foils that allow the passage of the electron beam and canbe supported in an opening in a container wall so that the vacuum withinthe chamber is maintained. As the electron beam passes through the metalwindow, the electrons are scattered to some extent with a consequentheating of the window. If a portion of the metal window is heated beyonda tolerable level as the beam is scanned along the window, thisoverheating causes oxidation and weakening of the metal and resultantpunctures through the metal. The necessary vacuum seal otherwiseprovided by the window obviously is destroyed upon the formation ofpunctures. In order to avoid such overheating of the window, theelectron intensity of the beam produced must be controlled rigidly, thuscausing beam generator operation below the potential output of thismachine.

Various steps have been taken to avoid the overheating described above.Cooling means such as blasts of air directed along the window are beingused, but they are not sufficient in themselves to dissipate enough heatto prevent adverse heating effects. Also, it has been proposed toprevent excessive localized heating by increasing greatly the size ofthe window so that the beam may be scanned over a large area.

While it is true that a larger window has a larger tolerable beamcurrent intensity, prior art arrangements having increased window sizeshave had severe disadvantages and limiting factors. This is becausethethin window materials require mechanical support structures of anexceedingly complex nature if these windows have large areas. Thesesupport structures cause gaps in the window area through which the beamcan pass and a resultant wastage of beam current when the beam impingeson the impermeable support structure. It is because of this thatconventional windows have a relatively large length compared to theirwidth. Window lengths, however, also are limited to support requirementsas attested to by the fact that accelerators with windows more than 72inches long are not known and many facilities have become unmanageablewith windows more than 48 inches long.

The necessarily narrow configuration of beam aperture windows alsolimits the efiectiveness of various multi-dimension beam scanningmethods designed to prevent overheating. Such methods do not restrictbeam movement to a single dimension, that is, along the length of thewindow, but rather provide for additional beam scanning at least alongthe transverse dimension of the window. Narrow window shapes greatlyimpede beam movement back and forth along the length of the window,regardless of transverse beam manipulations, without subsequent heatingof already heated locales and the consequent danger of overheating.

It is an object of this invention to provide a method for increasing theoutput of an electron beam generator by reducing the risk of windowoverheating due to the passage therethrough of a high current intensitybeam. This method takes advantage of a relatively large window area thatis a composite of plural windows, and provides for beam scanning overthe entire composite area without undue wastage of beam current due toimpingement of the beam on impermeable structure.

SUMMARY OF THE INVENTION This invention provides a method for increasingthe output of an electron accelerator by effecting this efiicientdistribution of available ionizing energy from the electron beamproduced in the evacuated container of the accelerator and directedtoward at least a pair of elongate, electron permeable windows arrangedin a side-by-side, spaced apart configuration in the wall of thecontainer. This method includes scanning the beam over a predeterminedtime period along the length of a first of the windows and theninstantaneously directing the beam from the first window to the secondof the windows. The beam then is scanned over a predetermined timeperiod along the length of the second of the windows and theninstantaneously directed from the second window to the first window. Thebeam scanning along the length of the first window is in a direction 180from the direction of the beam scanning along the length of theotherwindow. These steps may be repeated as a cycle as many times as desired.

DESCRIPTION OF THE DRAWINGS FIG. I is an isometric view illustratingschematically 'an electron accelerator that may be used to practice themethod of this invention;

FIG. 2 is a view taken along the line of sight identified by the arrow 2in FIG. 1;

FIG. 3 is a schematic representation of circuitry utilized inconjunction with theaccelerator of FIG. 1 and;

FIG. 4 is a composite representation of six electrical signals generatedby the circuitry of FIG. 3, these signals being plotted against time.

DETAILED DESCRIPTION OF THE INVENTION Referring now in detail to thedrawings, and in particular in FIGS. 1 and 2 thereof, the numeral 10denotes generally an accelerator useful in the practice of the method ofthis invention. This accelerator includes an elongate container 12defining an internal chamber 14. Proximate one end wall 16 of containerI2 is a hot cathode emitter 18. Remote from end wall 16, container 12includes a tapered portion 20 terminating in an end wall 22. End wall 22has formed therethrough a pair of elongated apertures in which aremounted electron permeable, spaced apart window members 24 and 26v As isconventional, window members 26 may be constructed of such electron beampermeable material as thin metal foil. The window members aresubstantially rectangular and are spaced apart with their longitudinalaxes lying substantially parallel to one another. Windows 24 and 26 aremounted in the apertures formed in the container end wall 22 such thatthey provide an air tight seal of these apertures so that a vacuum maybe maintained within chamber 14. Windows 24 and 26 are included in theanode structure of the accelerator. A great potential difference existsbetween this anode structure and cathode 18 such that electrons emittedby the cathode form an electron beam directed at the windows 24 and 26.

Control of the direction of this electron beam is provided by fourelectromagnetic scanning coils 28, 30, 32 and 34 positioned 90 apartabout the outer periphery of the container 12.

As illustrated, coils 28 and 30 are positioned to scan an electron beamemitted by cathode 18 in directions parallel to the Y-axis shown in FIG.2, that is, parallel to the longitudinal axes of windows 24 and 26.Coils 32 and 34 are positioned to scan the electron beam parallel to theX-axis shown in FIG. 2, that is, transverse to the longitudinal axes ofthe windows 24 and 26.

In the practice of the method of this invention, the scanning coils 28,30, 32 and 34 have the current passing therethrough and the resultantmagnetic field produced varied such that the electron beam extending thelength of chamber 14 has its direction varied in a particular manner.The particular scanning pattern that results provides that the currentintensity of the beam is maximized without danger of overheating of thewindows 24 and 26. The manner in which the electron beam is controlledmay be appreciated by reference to H6. 2. Let it be assumed that theelectron beam initially is directed by the scanning coils such that itimpinges upon window 26 at point 36. This point 36 is chosen arbitrarilyas the scanning of the electron beam in a closed path as explained belowis a continuous operation such that any point along the closed path maybe considered a starting or finishing point.

The current passing through scanning coils 32 and 34 is maintained at aconstant level such that no transverse movement parallel to the X-axisof the beam occurs. Simultaneously, the current passing through coils 28and 30 is varied such that the beam is canned over a predeterminedperiod of time along the broken line 38 as indicated by the arrows onthe line. Asthe beam reaches the end of window 26 remote from point 36,the current passing through coils 32 and 34 instantaneously is varied toa second constant level. This instantaneous variance produces aninstantaneous movement of the beam along the straight line 40 in adirection parallel to the X- axis. This instantaneous movement of thebeam from window 26 to window 24 precludes the wastage of'beam energydue to impingement of the beam on the portion of end wall 22 locatedbetween windows 24 and 26.

With the current in coil 32 and 34 maintained at the second constantlevel such that the beam impinges on window 24, the current in beams 28and 30 is varied such that the beam is scanned over a predeterminedperiod of time along the length of window 24 as indicated by the brokenline 42 and in a direction indicated by the arrows on line 42. It easilymay be appreciated that the direction in which the beam is scanned alongthe window 24 is 180 from the direction along which the beam is scannedover the window 26. When the beam scanning along window 24 reaches theend of this window, an instantaneous variance in the current passingthrough coils 32 and 34 causes an instantaneous movement of the beamalong the line 44 such that the beam is directed from window 24 towindow 26 and the starting point of the scanning cycle point 36. As withthe previous transverse scanning parallel to the X-axis, this movementfrom window 24 to window 26 occurs instantaneously and thus prevents thewastage of beam energy.

As explained above, it is by control of the current passing through thescanning coils that the beam scanning steps of the method of thisinvention are accomplished. The current through the coils is determinedby the operation of the circuitry shown schematically in FIG. 3 of thedrawings. It may be seen that a pulse generator 46 is connectedelectrically by means of a signal transmitting means 48 to a square wavegenerator 50 that sends output signals along a pair of signaltransmitting means 52 and 54 to a Y-axis driver 56 that in turn emitsalong signal transmitting means 58 the final current signal that isdirected to coils 28 and 30. As explained above, the current in thesecoils determines beam movement along the Y-axis.

The signal transmitted to the Y driver 56 along signal transmittingmeans 52 and 54 also is transmitted along signal transmitting means 60and 62, respectively, to the X driver 54 that produces current signalsthat are directed to coils 32 and 34 by means of signal transmittingmeans 66 and 68, respectively. The X driver is so termed because coils32 and 34 determine beam scanning movement parallel to the X axis ofFIG. 2.

As readily may be appreciated from the description of function set forthbelow, thepulse generator 46, square wave generator 50, Y driver 56 andX driver 64 are conventional electrical circuits that quite easily maybe constructed by one having ordinary skill in the art. The signaltransmitting means 48, 52, 54, 58, 60, 62, 66 and 68 may be suchconventional electrical components as conductive wire leads and thusconstitute no part of the instant invention.

Referring now to FIG. 4 of the drawings, the plot 4a represents thevoltage signal emitted by pulse generator 46 along signal transmittingmeans 48 to square wave generator 50. It may be seen that this signal ismerely a periodic pulse having a constant frequency. The output signalvoltage of the square wave generator 50 along the signal transmittingmeans 52 and 54 are represented by the plots 4b and 4c, respectively. Itmay be seen that plots 4b and 4c comprise square wave 180 out of phaseand having a frequency the same as the frequency of the pulses of plot40.

Plot M represents the saw toothed current signal emitted by the Y driver56 that receives voltage signals 4b and 4c. Current wave 4d is directedto both coils 28 and 30. that are connected in parallel, via the signaltransmitting means 58. As the current 4d progresses from a peak to avalley, magnetic fields given off by coils 28 and 30 caused the electronbeam produced in the accelerator 10 to be scanned in a first directionalong the length of one of the windows 24 or 26. As the current 4d thenprogresses from a valley to a peak, the electron beam is scanned in asecond direction, opposite to the first direction and along the lengthof the other of the windows.

X driver 64 receives each of the signals 4b and 4c and has outputcurrent signals 4e and 4f that are transmitted to coils 32 and 34 bymeans of conductive leads 66 and 68, respectively. The square wavecurrent values of the plots 4e and 4f are l out of phase and havepositive durations corresponding 7 to the durations of the current 4dalong either -a positive or a negative slope. It thus readily may beappreciated that the current in coils 32 and 34 remains constant duringthe periods when the current represented by the plot 4d is proceedingeither from a peak' to a valley or from a valley to a peak. Whenever thecurrent of the plot 4d reaches a transition point (peak or valley) thecurrent in coil 32 and 34 undergoes an instantaneous change resulting ininstantaneous movement of the electron beam parallel to the X-axis andfrom one of the windows 24 or 26 to the other. This arrangement thusinsures that no transverse scanning of the beam will occur whilelongitudinal scanning of the beam proceeds.

It thus may be seen that this invention provides a method of enhancingthe output of an electron accelerator by making possible the generationof an electron beam of high intensity without danger of localizedoverheating of the beam permeable window structure through which theelectron beam exits the accelerator structure. This method provides forbeam scanning between plural, electron permeable windows without thewastage of beam energy due to impingement of the beam on electron beamimpermeable structure.

We claim:

l. A method of effecting the efficient distribution of availableionizing energy from an electron beam produced in an evacuated containerand directed toward at least a pair of elongate electron permeablewindows arranged in a side-byside spaced apart configuration in a wallof said container, said method comprising the steps of: scanning saidbeam over a predetermined time period along the length of one of saidwindows, instantaneously directing said beam from said one window to theother of said windows, scanning said beam over a predetermined timeperiod along the length of said other window, and instantaneouslydirecting said beam from said other window to said one window.

2. The method of claim 1, further comprising the step of: repeating thesteps recited in Claim 1.

3. The method of claim 1, wherein beam scanning along the length of saidone window is a first direction and beam scanning along the length ofsaid other'window is in a second direction opposite to said firstdirection.

4. A method for increasing the output of an electron accelerator capableof producing an electron beam within an evacuated container, said beambeing directed at a pair of elongate electron permeable windows arrangedin a side-byside spaced apart configuration in a wall of said container,said accelerator including a first pair of beam scanning coilspositioned on opposite sides of said electron beam for scanning saidbeam along a path parallel to a first axis and a second pair of beamscanning coils positioned on opposite sides of said electron beam forscanning said beam along a path parallel to a second axis, each one ofsaid second pair of coils being spaced 90 from each one of said firstpair of coils such that said second axis is normal to said first axis,said method comprising the steps of: establishing a first varyingcurrent condition in said first pair of coils and a first constantcurrent condition in said second pair of coils, thereby scanning saidbeam in a first direction parallel to said first axis along the lengthof one of said windows; instantaneously establishing a second constantcurrent condition in said second pair of coils, thereby directing saidbeam in a path parallel to said second axis to the other of saidwindows; and establishing a second varying current condition in saidfirst pair of coils while maintaining said

1. A method of effecting the efficient distribution of availableionizing energy from an electron beam produced in an evacuated containerand directed toward at least a pair of elongate electron permeablewindows arranged in a side-by-side spaced apart configuration in a wallof said container, said method comprising the steps of: scanning saidbeam over a predetermined time period along the length of one of saidwindows, instantaneously directing said beam from said one window to theother of said windows, scanning said beam over a predetermined timeperiod along the length of said other window, and instantaneouslydirecting said beam from said other window to said one window.
 2. Themethod of claim 1, further comprising the step of: repeating the stepsrecited in Claim
 1. 3. The method of claim 1, wherein beam scanningalong the length of said one window is a first direction and beamscanning along the length of said other window is in a second directionopposite to said first direction.
 4. A method for increasing the outputof an electron accelerator capable of producing an electron beam withinan evacuated container, said beam being directed at a pair of elongateelectron permeable windows arranged in a side-by-side spaced apartconfiguration in a wall of said container, said accelerator including afirst pair of beam scanning coils positioned on opposite sides of saidelectron beam for scanning said beam along a path parallel to a firstaxis and a second pair of beam scanning coils positioned on oppositesides of said electron beam for scanning said beam along a path parallelto a second axis, each one of said second pair of coils being spaced 90*from each one of said first pair of coils such that said second axis isnormal to said first axis, said method comprising the steps of:establishing a first varying current condition in said first pair ofcoils and a first constant current condition in said second pair ofcoils, thereby scanning said beam in a first direction parallel to saidfirst axis along the length of one of said windows; instantaneouslyestablishing a second constant current condition in said second pair ofcoils, thereby directing said beam in a path parallel to said secondaxis to the other of said windows; and establishing a second varyingcurrent condition in said first pair of coils while maintaining saidsecond constant current condition in said second set of coils, therebyscanning said beam in a second direction parallel to said second axis.5. The method of claim 4, wherein said first direction is opposite tosaid second direction.
 6. The method of claim 4, further including thefurther steps of instantaneously re-establishing said first constantcurrent condition in said second set of coils, thereby redirecting tosaid one window said beam in a path parallel to said second axis andterminating the second varying current condition in said first pair ofcoils.